In subsea hydrocarbon production systems, a host production facility such as a shore based production facility or an offshore platform is typically connected to one or more subsea wells. Production pipelines connect each subsea well to the host production facility. Production pipelines are used for conveying extracted hydrocarbons back to the host production facility. In addition to the production pipelines, it is necessary to provide each subsea well with certain services. These may include electric power, data transfer, hydraulic fluids and wellstream service fluids, such as chemical additives. These four services are normally provided in a subsea umbilical.
Referring to FIG. 1 herein, there is illustrated schematically a network of subsea wells connected to a host production facility 1. Several subsea umbilicals 2, 3, 4, 5 are connected to the host production facility 1. Each umbilical connects several subsea wells (shown in FIG. 1 as circles) to the host production facility 1. Taking umbilical 5 as an example, this connects subsea wells 7, 8 and 9 to the host production facility 1. Note that each subsea well, for example subsea well 7, could comprise several physical wells. In addition to the umbilicals, hydrocarbon-carrying pipelines are also shown as thick black lines that follow substantially the same route as the umbilicals 2, 3, 4, 5.
FIG. 2 is for the purposes of illustration only to show the services provided in an umbilical. An umbilical 2 typically includes several hydraulic fluid conduits 10 (four are shown in FIG. 2), for providing hydraulic power for opening and closing valves. Furthermore, an umbilical typically includes several wellstream service fluid conduits 11 (four are shown in FIG. 2) for carrying wellstream service fluids of different compositions, depending on the wellstream composition and the purpose of the wellstream fluid. Seven electrical cables 12 are shown in FIG. 2, which provide electric power to subsea control modules. The subsea control modules in turn provide hydraulic power to subsea wells. Furthermore, four data transfer cable 13 bundles are shown in FIG. 2. Of course, these numbers are illustrative only, and different umbilicals may have different numbers of hydraulic fluid conduits 10, wellstream service fluid conduits 11, electrical cables 12 and data transfer cables 13. This depends on various factors, such as the number of subsea wells served by the umbilical, the distance that the umbilical must traverse and so on. A protective sheath 14 is also provided.
Subsea umbilical 2 connects the subsea wells 7, 8 and 9 to the host production facility 1 in series, typically by ‘daisy-chaining’. In other words, in order to provide electric power and data transfer between the host production facility and subsea wells, these services must pass subsea wells 7 and 8. A typical subsea umbilical may serve 12 or more subsea wells, but there are practical limits on the number of subsea wells that can be served by a subsea umbilical.
As control systems for subsea wells become more sophisticated, the electric power and data transfer requirements for each subsea well increase. In time, this will either reduce the number of subsea wells that can be connected to each subsea umbilical, or require subsea umbilicals with a larger capacity to transfer electric power and data. This could greatly increase the costs of subsea umbilicals.
Another problem with existing subsea umbilicals is that the cost of extending a subsea umbilical to a new subsea well in a new marginal prospect can be prohibitively expensive, especially if the subsea well must be connected to a host facility with a new umbilical.
A further problem associated with existing subsea umbilicals is that when a subsea umbilical develops a fault, it can affect all of the subsea wells ‘downstream’ of the fault, and so repairs and upgrades can be expensive in terms of lost production.
Existing subsea umbilicals use alternating current. Each power transmission cable in a subsea umbilical acts as a capacitor, and so where alternating current is used the capacitance properties appear in parallel with the electrical load. This leads to losses in power transmission, which limit the practical length of a subsea umbilical to around 150 km.